The invention set forth in this specification pertains to a new and improved method and apparatus for causing necrosis of neoplasm by hyperthermia.
Aberrant cells within the body, such as are frequently referred to as neoplasms, are the subject of a great deal of concern to society and civilization because of the very undesirable consequences of various growths such as are commonly referred to as tumors, cancers and the like. Tremendous sums of money have been and will continue to be spent in connection with the problem of causing necrosis of neoplasm with a minimal detrimental effect to a person at a minimal cost. It is considered that no one approach to this problem of causing necrosis of neoplasms will ever be completely satisfactory for use with all different types of neoplasms.
It has been recognized that certain types of neoplasms can be effectively necrosed as the result of the use of hyperthermia. In the past, the heating used in this type of treatment has been achieved either through the use of the electromagnetic radiation associated with a radio frequency or microwave field or through the use of an alternating magnetic field such as is associated with induction heating. It is considered that an understanding of the present invention requires an understanding as to essential differences between these two distinctly different types of heating.
A radio frequency or microwave field can only be utilized in causing heating at or adjacent to the surface of a body because of the tendency of the body to absorb radio frequency electromagnetic radiation. The depth to which such radiation will penetrate the body will vary depending upon the frequency of the radiation. Such body tissue heating as is caused by radio frequency radiation has two principal components: eddy current heating and dielectric heating. Both result from the electromagnetic field employed passing through the tissue. Effective heating of this type can be achieved in the absence of any foreign material in tissue being heated.
As opposed to this, induction heating of body tissue as a result of the presence of a material having hysteresis characteristics may be referred to or at least considered as hysteresis heating. It is based upon the use of a magnetic field which is normally presumed to be unattenuated by tissue. Hence, induction heating can reach any desired depth or level beneath the surface of tissue. In order to achieve this manner of heating in tissue, it is necessary to locate a material exhibiting magnetic hysteresis at or about the location where heat is desired. Any such induction-hysteresis heating will be accompanied by some eddy current and dielectric heating.
The amount of eddy current and dielectric heating can be minimized by utilizing a comparatively low frequency alternating magnetic field. The amount of such heating varies directly with the square of the frequency of the field. As opposed to this, the amount of hysteresis heating forming the operative "part" of induction heating for use with the present invention varies directly with the frequency. Thus, in general, the lower the frequency of the field the less the amount of undesired eddy current heating achieved relative to the amount of the desired hysteresis heating achieved. The frequency used, however, should not be so low as to preclude the achievement of a desired degree of hysteresis heating.
The limited amount that a radio frequency field can normally penetrate body tissue has been and remains an important factor limiting the use of this type of heating in the treatment of various different neoplasms. Obviously, this radio frequency heating can not be utilized effectively in causing hyperthermia in aberrant growths which are sufficiently beneath the surface of tissue so as not to be reached by a radio frequency field. The use of such a field to cause hyperthermia at or adjacent to the surface of the skin or at or adjacent to the interior of an incision within the body is limited by another important factor--the problem of selectively concentrating the heat developed so that the neoplasm is heated to a point sufficient to cause necrosis without there being a related or corresponding heating of adjacent normal tissue.
This can be easily illustrated by referring to specific temperatures such as have previously been recognized to be important in causing necrosis as a result of hyperthermia. It is usually considered that necrosis will not be caused by the exposure of most body tissue to a temperature of about 40.degree. C. for a reasonably prolonged period. It is also generally considered that if commonly found body tissue--including neoplasms--are held at a temperature of at least 42.degree. C. for a reasonably prolonged period that such cells or tissue will be destroyed if the period is adequately long for the purpose.
It is also generally conceded that commonly encountered cells and tissue--including neoplasms--will be necrosed by exposure to a temperature of from about 60 degrees centigrade or, of course, a higher temperature for a very minimal or limited, practically instantaneous time period. It should be noted that body tissue should not normally be subjected to a temperature reasonably approaching 100.degree. C. for even a very short, instantaneous time period because of the possibility of the vaporization of water in the body and the attendant possibility of damage being caused by the sudden expansion of vaporization of water. As a practical matter, it is considered that internally the body should never be heated past 90 degrees centigrade to avoid any possibility of such vaporization.
Apparently, the relationship between the time that neoplasm or other body growth need be exposed to a specific temperature to cause necrosis and the time necessary for such exposure to cause necrosis are not related in exactly usual time-temperature reactions. Further, certain parts of the body or neoplasms are more susceptible to necrosis at an elevated temperature than other parts of the body and/or other certain neoplasms. Because of these factors, it is impossible to give exact relationships between the time and the temperature necessary to accomplish necrosis and, in particular, necrosis of neoplasms. Another factor which will be important in considering the amount of hyperthermia to cause necrosis in neoplasm will involve the tendency of the body and/or a particular portion of the body to serve to at least a degree as a heat insulator so as to tend to concentrate heat developed, as for example, at a specific source.
With radio frequency or microwave hyperthermia the difficulty in confining the field necessary to cause necrosis of a specific area or region will normally cause necrosis of healthy tissue in an adjacent area or region. While to a degree this can be combatted through the use of specialized electrodes, it is not considered that it is possible to adequately control radio frequency or microwave heating in many applications so a to avoid detrimental damage to normal cells or tissue. While it may be possible to improve the concentration of the heating effects achieved with radio frequency or microwave heating through the implantation of a metallic conductor or another similar material in a specific area where concentrated heating is desired, it is considered that such a use of a conductor is not always advantageous because such a conductor will not improve the depth of pentration of tissue by a radio frequency field.
The use of induction heating in causing necrosis of neoplasm is considered desirable in overcoming several problems involved in utilizing a radio frequency field to cause hyperthermia leading to necrosis of neoplasm as noted in the preceding. When induction heating is used for this purpose, it is necessary to utilize an invasive technique to locate a material having a magnetic permeability greater than unity and capable of exibiting hysteresis losses--for example, ferromagnetic particles such as a conventional ferrite or conventional iron or steel powders--in or immediately adjacent to the neoplastic growth. The use of such a magnetic material in an alternating magnetic field results in hysteresis heating within the magnetic material itself. This localized or focused hysteresis heating overcomes the problem of containing or limiting the heated area obtained with a radio frequency field. Because of the penetration of a low frequency magnetic field with respect to body tissue, it is possible to use a magnetic material well beneath the surface of the body.
From this it is believed that it will be apparent that the utilization of induction hysteresis heating in connection with the hyperthermia of neoplasms has a great deal of advantageous potential. Unfortunately, it is not considered that the prior efforts for accomplishing necrosis of neoplastic growth by induction heating have been sufficiently effective and desirable for this purpose. It is considered that several factors may be important in connection with this.
One of these concerns the lack of availability of induction heating equipment which is specifically adopted for use in treating a body such as a human or animal body in which magnetic material has been implanted so as to produce localized heating. In connection with this, it is noted that prior induction heating coils have not normally been designed with the intent that they be utilized to provide a very intense AC magnetic field over a comparatively large volume so as to cause heating in a specific, very limited area or region coming within the "scope" of the complete magnetic field produced by the coil in which material capable of hysteresis heating is located. It is considered that this factor has led to a degree of inefficiency of prior induction heating coils when such coils have been utilized in conjunction with body tissue so as to cause inductuon heating of materials or particles embedded within such tissue.
It is considered however, that the prior procedures for accomplishing necrosis of neoplastic growths by hysteresis heating have also been comparatively unsatisfactory for other reasons relating to the technique or procedure followed in implanting the magnetic material or particles used. In general, such materials have not been utilized in such a manner as to be effectively immobilized either within or immediately adjacent to a neoplasm so as to be capable of providing a localized heating without danger of the magnetic material used moving from its initial location within the body. Any such movement would be highly disadvantageous because of the possibility of undesired interference with the normal operation of the body.